G-36 Tightbore

I was looking to get a tightbore for my new gun but I'm not really sure where to start.

http://www.airsoftgi.com/product_info.php?cPath=139_24_160&products_id=4878

Ok, so how tight of a Tightbore do you want?

This would be a good place to start. You're looking at around a barrel length of 357mm but i'm not sure if you may need a barrel extension or not with a TBB. Bore is up to you, if you'll be using top-quality precision BB's you could use a 6.01 or if something a little more general a 6.03/4/5.

@Nova

iLiveForThis99 is correct. You will need a length around 357mm. For the bore tightness I personally recommend 6.03 for any length because as the bb travels through the barrel a pocket of air acts as a buffer between the bb and bore along the bb's entire circumference. The reason for this is because of the enormous amounts of airpressure behind the bb.
If you had a bore-barrel (6.00) the bb would have too much friction and the hop up would be rendered pointless.